For use in various foods, food additives, chemical materials, etc., organic acids have been long produced by fermentation. In recent years, organic acids have been evaluated as resources highly available as raw materials for fuels and chemical materials useful in the development of new environmentally friendly renewable energy and chemicals.
Fermentation for the production of organic acids is generally conducted in a batch manner. The production of organic acids by fermentation is disclosed in U.S. Pat. Nos. 5,503,750 and 5,766,439. According to batch fermentation, microorganisms for producing an organic acid of interest are grown in a bioreactor where a limited supply of nutrients for the growth, such as sugars, a nitrogen source, minerals, etc, are fed simultaneously. Since the yield of organic acids from a limited supply of sugars is limited, a high concentration of the sugars is initially employed to give a greater amount of the organic acids in a batch culture. However, a high concentration of sugars acts as a cause of inhibiting the growth of microorganisms to decrease fermentation rate. Fed-batch culture is suggested as an alternative to solve the problem.
Fed-batch culture is an operational technique in biotechnological processes where batch culture is initially conducted at a suitable sugar concentration, and then high concentration sugar is subsequently supplied to maintain low concentration of sugar in the culture medium. In regard to this technique, there are various fed-batch culture methods that are dependent on when, how, and how much to feed sugars. The growth rate of microorganisms and product yield can be affected according to each methods.
Sugar controlling techniques known to date include an manually feeding sugar at a constant rate determined according to the growth pattern analysis of microorganisms, and automatically feeding sugar in response to the information on DO (dissolved oxygen) or pH (hydrogen exponent). When employing the former technique, fed-batch culture may improve in productivity and yield, compared to batch culture, but may be inefficient because sugar cannot be fed accurately in concert with microorganisms growth and sugar consumption. Further, the operator should continually observe the feeding amount of sugar.
As for the latter technique, pH stat or OD stat fed-batch fermentation is widely employed for culturing aerobic microorganisms such as E. coli because it causes sugar concentrations in the culture to fluctuate little, and matches the profile of microorganisms growth. Particularly, pH-stat fed-batch culture was developed in consideration of the pH drop phenomenon that occurs as aerobic microorganisms such as E. coli metabolizes sugars, and is designed to feed sugar at a concentration that is minimal but not decreases the activity of microorganisms. In the pH-stat fed-batch mode, a desired pH is set, and a small amount of sugar is fed when the pH of the culture exceeds the allowance of the set value. As a small amount of sugar is metabolized, the pH slightly decreases. If the sugar is completely consumed, the pH of the culture again increases, and thus sugar is fed. This cycle is continued until the product is obtained in a desired amount. Upon fermentation for organic acid production, however, the culture does not undergo the sugar consumption-induced slight increase of pH, but constantly decreases in pH with the production of organic acids. Therefore, conventional fed-batch culture, such as pH stat, cannot be applied to fermentation for the production of organic acids.